I've reinstalled the HG612 with B030SP06_unlockedgui-nobtagent firmware and checked the memory usage (all values are kB)
Used Free Shared Buffers Cached
---- ---- ------ ------- ------
Not connected, with GUI: 22428 6328 0 2188 6480
Not connected, GUI killed: 21840 6904 0 2188 6484
Connected, with GUI 22844 5900 0 2244 6700
Connected, GUI killed 22220 6524 0 2244 6704
In all cases remote devices failed to receive internet data. So the problem isn't the presence of the GUI as such, nor would it seem to be a memory issue.
Having killed the GUI, I'd like to try restarting it, but I'm not sure how to do that. The command appears to be /bin/web, but however I execute that command it doesn't work.
[*** Sorry about the formatting - it looked OK on preview]